Electrical tilt switches and like devices operate to switch electrical circuits either ON or OFF as a function of the angle of inclination of the switch. See, for example, U.S. patent application Ser. No. 07/822,641 filed Jan. 21, 1992 now U.S. Pat. No. 5,209,343 by Robert P. Romano, et al. and entitled "ELECTRICAL TILT SWITCH" and assigned to the assignee herein. This application discloses various switch configurations which can be employed with the present invention. Such switches normally include a free moving electrically conductive element that contacts at least two terminals when the conductive element is biased to an operating position by gravity. A well known form of the electrical tilt switch is the mercury switch. In a typical mercury switch, a glob of mercury moves freely within a housing. As the housing is inclined, gravity pulls the glob of mercury to one end of the housing where it completes an electrical circuit.
Mercury tilt switches are fairly easy to manufacture, however, due to environmental concerns, it is becoming increasingly difficult to manufacture any product that includes mercury. Mercury is a highly toxic substance. As such, there exists a large number of federal, state and local guidelines controlling the use, storage and disposal of mercury. The increase in governmental regulation has increased the cost of manufacturing mercury switches to a point where alternative non-mercury tilt switches have become more competitive.
When manufacturing a tilt switch without mercury, a substitute free moving conductive element must be used. A common substitute is a single metal ball. Tilt switches utilizing metal balls in place of globs of mercury are exemplified in U.S. Pat. Nos. 4,628,160 to Canevari; 4,467,154 to Hill; 4,450,326 to Ledger; and 3,706,867 to Raud, et al. The use of a metal ball to complete an electric circuit is a simple and inexpensive way to create a tilt switch. However, metal balls do have certain inherent disadvantages. A metal ball contacts a flat surface only along its tangent. Consequently, in many prior art switches that use flat contact points, only a small area of the metal ball is in actual electrically conductive contact within the switch. Adversely, with mercury switches, the mercury glob would envelope a terminal as it contacted it, resulting in a large surface area through which electricity could be conducted. The comparatively small surface area of a metal ball, through which electricity can be conducted, has made metal ball tilt switches generally less reliable than mercury switches.
Another disadvantage of conventional metal ball tilt switches is that when a metal ball does contact a terminal, the resulting electrical coupling across the contact area is poor. In a mercury switch, the mercury glob would flow into any pit or void it encountered on a terminal, creating a good electrical coupling. However, with conventional metal ball tilt switches, the metal ball is unable to conduct electricity across any pits or voids that exist on either the surface of the terminal or the metal ball itself. Since electricity passes through the metal ball from the terminal it is contacting, arcing can occur across any void in the contact surface. The arching may cause pitting or corrosion on both the metal ball and the terminal, reducing the conductivity of both surfaces.
In an attempt to improve the functional reliability of metal ball tilt switches, switches have been created that use multiple balls. By using multiple balls the points of contact between the balls and the terminals is increased, thereby increasing the reliability of the switch. Furthermore, by using multiple balls within a switch, the forward lying balls are pressed against the terminal contacts of the switch, by the weight of the rearward balls. The bias provided to the forward balls by the rearward balls create an improved electrical coupling of the forward balls with the switch terminals, thereby increasing the overall reliability of the switch. Prior art switches that utilize multiple metal balls are exemplified by U.S. Pat. No. 2,107,570 to Hobbs and U.S. Pat. No. 2,228,456 to Hobbs.
A disadvantage of many prior art tilt switch that use multiple balls, is that one never knows which ball or balls with actually contact the terminals within the switch. Consequently, each of the balls must be manufactured to be highly conductive and corrosive resistant so as to provide a proper electrical contact. Manufacturing a multitude of such conductive and corrosive resistent balls adds greatly to the cost of such prior art tilt switches. Another disadvantage of some switches that utilize multiple balls is that the switches are highly sensitive to vibrations. As a switch with multiple balls experiences vibrations, the position of the multiple balls within the switch may change as the various balls roll over each other or reorient themselves to the forces of gravity. Such movements of the balls may cause slight changes in the overall impedance and resistance of the switch, thereby making the switch a poor choice for use with sensitive circuitry. Furthermore, as a tilt switch is inclined, the metal balls roll toward the terminals within the switch. However, as the balls strike the terminals, the balls may bounce a slight distance away from the terminal. Consequently, the contact of the ball bouncing against a terminal produces a short pulsed signal that can adversely effect some sensitive circuitry.
It is therefore an objective of the present invention to provide a more reliable tilt switch that utilizes a plurality of ball contacts, wherein only one ball contact is of precision manufacture, thereby reducing the cost of producing the tilt switch.
It is yet another object of the present invention to create a more reliable tilt switch that is list susceptible to creating false or changing signals that can adversely effect sensitive circuitry.